390 CHEMICAL DYNAMICS 



leucin may be recovered from a sterile suspension of casein in 

 pure water and that arginin may be recovered from a solution 

 of protamin sulphate in pure water, both after the lapse of a 

 year or more (133); other examples of the slow autohydrolysis 

 of proteins in pure water might be adduced (133) (131). In a 

 few cases, however, the autohydrolysis occurs at a readily meas- 

 ureable velocity. I have found (110) that the velocity-constant 

 of the hydrolysis of casein in milk at 36 degrees (in the presence 

 of excess of toluol) is 0.000546, common logarithms being em- 

 ployed and the time expressed in hours (using the monomolecular 

 formula, Cf. previous chapter equation (i)). For a 2.8 per cent 

 solution of casein in NaOH, carefully neutralized to litmus and 

 therefore containing H + and OH' ions in the concentrations in 

 which they exist in pure water, the velocity-constant at 36 de- 

 grees, similarly estimated, proved to be 0.000518. In the first 

 experiment the extent of hydrolysis was estimated by deter- 

 mining the residue of undigested casein after 32 days, in the 

 second experiment the undigested residue was determined after 

 20 days. Now in the second experiment, at all events, no fer- 

 ments were present and in both experiments the solutions were 

 almost exactly neutral. We must therefore regard the observed 

 hydrolysis as being not due to catalysors but to the action of the 

 solvent water itself or of its ions.* Expressed in numerical 

 terms, the above-cited results mean that in absolutely neutral 

 solution, in the absence of any proteolytic enzymes, one-half of 

 a caseinate of sodium or calcium is hydrolysed in about 24 days. 



The hydrolysis of neutral caseinates by trypsin or pepsin 

 affords, therefore, an unusually favorable example, among pro- 

 tein reactions, of the action of an enzyme in accelerating an 

 already progressing reaction. 



* Tt might be inferred from these results that normal milk contains no 

 proteolytic enzymes. This inference would not be altogether a safe one, how- 

 ever, since the milk which was employed was obtained in the open market and 

 there is reason to suspect that it had been manipulated in a manner which, it 

 is possible, may have destroyed pre-existing enzymes. No difficulty was en- 

 countered in keeping this milk sterile, throughout the course of the experi- 

 ments, by the simple addition of toluol. When milk obtained from the 

 University Experimental Farm was employed, however, which had not been 

 pasteurized, it was found impossible to keep the milk sterile, for the length 

 of time required, without employing, as sterilizing agents, substances which 

 might conceivably destroy or injure proteolytic enzymes. 



